A typical conventional contact type image sensor which is applied to a facsimile document reading device will be explained hereinafter with reference to FIGS. 2(a) and (b), in which the regions shown in the cross-section in FIG. 2(a) with different forms of hatching are shown in the same framing pattern in the plan view of FIG. 2(b). In this conventional contact type image sensor, a gold film is attached on an insulating substrate 11 by means of screen process printing and successive baking, and is then processed by a photoetching method so as to form wires 12. A film of chromium is then deposited and patterned by photoetching such as to form individual electrodes 13 and leads 14. The leads 14 are formed such that they electrically connect the wires 12 to the electrodes 13. Subsequently, a film of intrinsic amorphous hydrogenated silicon (hereafter referred to as an "a-Si") 15 formed from silane gas mixed with hydrogen by means of plasma CVD method is selectively deposited through an appropriate mask in such a manner that the film 15 covers the individual electrodes 13. Finally, a film of indium-doped tin oxide (ITO) is deposited through a mask to form a common electrode 16 in such a manner that it overlies the individual electrodes 13. The light incident from a document to be read enters the sensor from the side of the common electrode 16 and penetrates the ITO film which is light transmissive.
In such an image sensor, however, since the common electrode 16 is formed by means of deposition through a mask and so results in low accuracy, the distance of overlap between the edge of the common electrode 16 and the edge of each individual electrode 13, shown by x in FIG. 2(b), tends to vary. Moreover, the portion of each lead 14 which underlies the common electrode 16, i.e., the unhatched area s is also involved in photoelectric conversion. As a consequence, even if the area of each individual electrode 13 is made constant, the photoelectric output of each photo detecting element may vary if the area s is changed. As a result, the capacity to read picture element may change in every element or in every lot, and this eventually leads to a reduction in the production yield.
In order to increase the dimensional accuracy of the common electrode 16, the ITO which has been selectively deposited may instead be patterned by the photoetching method so that it has a desired width. In that case, however, the etchant used may contact the a-Si film 15 and deteriorate the quality of the film. This also results in a decrease in the yield of production.
In view of the above-described problems, an object of the present invention is to provide a contact-type image sensor which has better uniformity in the photoelectric output of each light-receiving element and which can be readily manufactured with high production yield.